This invention relates to the predominantly para chlorination of phenols and phenoxyacetic acid employing sulfuryl chloride in the presence of elemental sulfur or certain other sulfur compounds as a catalyst.
The chlorination of aromatic compounds with sulfuryl chloride is generally known. For example, DuBois in Z. F. Chem. 705 (1866) reported that treatment of molten phenol with an equal molar amount of sulfuryl chloride yields only p-chlorophenol. Sulfuryl chloride was first reported as a chlorinating agent for o-cresol by Sah and Anderson in J. Am. Chem. Soc. 63, 3164 (1941). Their data showed that o-cresol reacted with sulfuryl chloride to yield 84% of the 4-chloro-o-cresol.
Sullivan in U.S. Pat. No. 2,777,002 and British Pat. No. 948,601 reported that phenols, especially those containing hydrogen or an ortho-para direction group in the ortho position, may be converted to the p-halophenol in yields as high as 95% at temperatures not exceeding 75.degree. C. by the use of 1.1 mols of sulfuryl halide per mol of phenol in the presence of 0.5 to 2% of a Friedel-Crafts metallic halide catalyst. Haesler et al in German Pat. No. 1,203,275 further improved the work of Sullivan by finding that if the Friedel-Crafts catalysts was introduced into the reaction as a powdered metal rather than as a salt, the yield of the 4-chloro-o-cresol was increased. Throughout these reactions, the rate of chlorination has been low. For instance, Sullivan shows essentially complete reaction at temperatures of 10.degree. to 30.degree. C. in no less than 8 hours.
The chlorination of benzene with sulfuryl chloride in the presence of a catalyst of sulfur, sulfur chloride and mixtures of AlCl.sub.3 and sulfur chloride is described by Silberrad in J. Chem. Soc., 119, 2029 (1921) and British Pat. No. 193,200. Such chlorinations do not show the selectivity of the present invention.
U.S. Pat. No. 3,920,757 describes the chlorination of phenolic types of aromatic compounds with sulfuryl chloride where the rate of reaction is substantially increased and the selectivity of the reaction toward para chlorination is enhanced by conducting the reaction in the presence of a small but effective amount of certain metal salt-organic sulfur catalysts. Patentee, however, found with his cocatalysts that the temperature was critical and should be maintained below 60.degree. C. where para chlorination was desired.
The presently employed technology to produce 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 2,4,5-trichlorophenol (2,4,5-TCP) commercially involves the dechlorination of 1,2,4,5-tetrachlorobenzene with aqueous sodium hydroxide or anhydrous sodium hydroxide in the presence of alcohols under elevated temperatures and pressures to form 2,4,5-TCP according to the reaction: ##STR2## Under the conditions of alkalinity, temperature and pressure, this process produces between 1 to 100 parts per million of the highly toxic teratogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) according to the reaction: ##STR3##
In the production of 2,4,5-TCP by this process, the TCDD produced is usually separated by careful distillation, extraction or by adsorption on activated carbon. However, the TCDD is claimed to be extremely toxic and cannot readily be disposed of. TCDD is a highly stable compound which is difficult to destroy chemically. Waste disposal by burying is unsatisfactory. Its destruction by incineration is a possibility which is presently being investigated. However, incineration of such a highly chlorinated compound is difficult and presents the possibility of contamination of either the scrubber liquor, the exhaust gas, or any solid residues.
The production of 2,4,5-T from 2,4,5-TCP is by reaction with monochloroacetic acid according to the reaction: ##STR4## Conditions of this reaction are mild in comparison to the hydrolysis step to produce 2,4,5-TCP. Consequently no measurable new formation of TCDD can be observed in the latter reaction.